A Page Description Language (PDL) is a language that describes the contents of a document at a higher level than an actual output bitmap. A PDL script defines page items, or graphical objects, independently of render technology, such as printers or display devices, so that a document's appearance is consistent regardless of the specific render device used. Graphical objects may include lines, rectangles, arcs, images and text. Examples of PDLs include Adobe® PostScript®, PDF (Portable Document Format), and Hewlett Packard® PCL (Printer Control Language).
PDL compression is commonly achieved by decimating images and processing PDL commands to remove redundant colors, useless clips, degenerate strokes and unused objects. For example, Adobe® Acrobat® Professional's PDF Optimizer compresses PDF by:                Downsampling color and grayscale images using JPEG, ZIP, run length, JBIG2, or CCITT compression;        Unembedding fonts;        Flattening transparency;        Discarding unused objects;        Converting smooth lines to curves;        Detecting and merging image fragments;        Using Flate to compress uncompressed streams;        Removing invalid bookmarks and links; and        Removing unreferenced named destinations.The application PDF Compress by VeryPDF Inc. is another software application that compresses PDF files in a similar way, by:        Applying JPEG2000, JPEG, or Flate compression for color images;        Applying CCITT Group4, or Flate compression for grayscale images; and        Removing unused objects.        
On-line catalogues that are designed in products such as QuarkXpress® or Adobe® Illustrator® are often of the order of 1 MB per page. This size is prohibitive for display of catalogues on websites. The compression applications described above do not achieve sufficient compression. A more aggressive algorithm is required to generate files of a size suitable for web download, which also preserves the quality of certain graphical objects, referred to as key items.
Text is an example of a class of key items. The previous methods mentioned above do not sufficiently preserve the following text attributes:                Visual appearance: Is the text crisp and clear to read?        Readability: Are text kerning, leading and word spacing preserved?        Searchability: Is the text searchable in viewer applications?        Scalability: Is the visual appearance of the text retained with zooming into and out of the text?        Extractability: Is the text extractable from the document, while retaining all its attributes?        
Other classes of key items include PDF graphical objects that are intended to represent real-world objects. Users of computer aided design (CAD) documents which include such objects often require information about the scale and units of measurement of the corresponding real-world objects and their relationship to units in PDF user space. The extractability of measurement information from these PDF graphical objects must be preserved by the compression method. This information enables users of viewer applications to perform measurements that yield results in the units intended by the creator of the original document.